Electrical liquid level gauge



y 1966 TERUO KUMANO ETAL 3,260,903

ELECTRICAL LIQUID LEVEL GAUGE Filed July 24, 1964 United States Patent O3,260,903 ELECTRICAL LIQUID LEVEL GAUGE Teruo Kumano and Tatsuo Nozaki,Tokyo, Japan, assigngrs to Nippon Denshi Sokki KabushikiKaisha, Tokyo,

apan

Filed July 24, 1964, Ser. No. 384,985 Claims priority, applicationJapan, July 30, 1963,

s/s4,s71 1 Claim. (Cl. 317246) This invention relates in general to anelectrical liquid level gauge and more particularly to a spacer formeasuring electrodes for use in such a gauge.

As will be well known, an electrical liquid level gauge includes a pairof cylindrical metallic electrodes disposed coaxially with each other toform a measuring annular space having a predetermined fixed widththerebetween. A level of liquid to be measured can be determined byelectrically measuring an electrostatic capacity of that portion of themeasuring space which is filled with the liquid in any conventionalmanner, in the assumption that the liquid has a constant dielectricconstant at a measuring temperature. Therefore, in order for theelectrical liquid level gauge to consistently indicate a precise readingfor a liquid level, a spacing between the pair of electrodes must alwaysbe held exactly constant. To this end, a spacer or spacers is or areusually used to be sandwiched in between the two electrodes. The spacerused is required to maintain the predetermined exact distance betweenthe electrodes, not to interfere with any variation in the liquid levelto be measured, and to be coupled to the electrodes in such a mannerthat its weakening effect on the electrodes is minimum. Of course, thematerial of the spacer should resist to any chemical attack due to theliquid to be measured.

The chief object of the invention is to provide an improved spacer forseparating a pair of coaxial cylindrical electrodes for use in anelectrical liquid level gauge which spacer meet the requirements asdescribed in the preceding paragraph.

Another object of the invention is to improve an electrical liquid levelgauge by rendering the same substantially insensible to externalvibrations.

According to the teachings of the invention a spacer for separating apair of coaxial cylindrical electrodes from each other in an electricalliquid level gauge is composed of an electrically insulating materialselected from the group consisting of tetrafiuoroethylene, copolymers ofhexafiuoropropylene and tetrafluoroethylene, polyethylenes, polyamides,polyesters, polycarbonates, polyvinyl chlorides, glasses and ceramicsdependent upon the type and temperature of a liquid to be measured. Thespacer may be in the form of a rivet or a short rod having apredetermined fixed length. On the other hand, one of two coaxialcylindrical electrodes, for example, the internal electrode is providedon its external surface with a plurality of small recesses complementaryin shape to the spacer and disposed on at least one predetermined linesuch as a helical line. Then the spacers are planted in the recesses onthe internal cylindrical electrode so as to project beyond the surfaceof the internal electrode by a predetermined fixed length and theexternal cylindrical electrode is fitted onto the internal electrodewhereupon the spacers serve to maintain both the electrodes apredetermined fixed distance determined by the length of that portion ofeach spacer projecting from the internal surface of the internalelectrode.

The invention will become more readily apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 shows a fragmental view of one measuring electrode according tothe teachings of the invention; and

Patented July 12, 1966 ice FIG. 2 shows a cross sectional view of astructure of electrodes embodying the teachings of the invention.

In practicing the invention, one of two cylindrical electrodes made ofany suitable metal such as aluminum is provided on that surface thereoffacing the other electrode with a plurality of small recesses in apredetermined pattern. Such recesses may be preferably disposed atsubstantially equal intervals on a helical line on the externalcylindrical surface of the internal electrode as shown in FIG. 1. InFIG. 1, the reference numeral 1 designates such an internal cylindricalelectrode whose external surface has a plurality of small circularrecesses 20, 2b, 2c and 2d disposed at substantially equal intervals ona helical line denoted by dotted line thereon. It should be understoodthat the recesses may be, if desired, disposed in any other pattern, forexample, on three generatrices for external cylindrical surface of theinternal electrode positioned at angular distances of 120 degrees. Therecesses as viewed from the one end of the electrode are preferablydisposed at substantially equal angular intervals on the profile of theelectrode.

A spacer which is fitted into individual one of the recesses is composedof any of electrically insulating materials as will be describedhereinafter. As shown in FIG. 2, the spacer 3a, 3b or 30 can be in theform of a rivet whose stem is complementary in shape to the recess. Itis to be noted that that ortion of the spacer projecting beyond thesurface of the electrode 1, in this case the head portion of each rivet3a, 3b or 30 should have a predetermined fixed common lengthcorresponding to a predetermined magnitude of spacing between theexternal and internal electrodes in their assembled positions.

During or after the spacers are fitted into the respective recesses onthe internal electrode, an external electrode 4 is fitted onto theinternal electrode 1 whereby both electrodes have the spacers sandwichedand maintained therebetween to form a unitary structure as shown in FIG.2.

According to the teachings of the invention, a material for the spacerscan be properly selected dependent upon the type and temperature of aliquid to be measured. The following materials have been foundpreferable to be used as the spacers. Tetra-fiuoroethylene availableunder the name Teflon can be used for liquified nitrogen, oxygen, argon,ammonia, butane and propane and petroleum at a temperature within arange of from +200 to -270 C. and may be advantageously employed tomeasure a liquid level of any combustible. Copolymers ofhexafiuoropropylene and tetrafluoroethylene such as available under thename PEP can be used for liquified nitrogen, oxygen, argon, chlorine,ammonia, butane and propane, and petroleum at a temperature between +200and -270 C. and are characterized by chemical stability. Polyethylenecan be used for liquified nitrogen, argon and carbon dioxide and waterat a temperature between +70 and -200 C. Poly-amides or nylon can beused for liquified nitrogen, and argon and various oils at a temperatureranging from to --200 C. Polyesters available under the names Dacron andTerylene can be used for liquified nitrogen, argon and carbon dioxide,Water and petroleum at a temperature ranging from +100 to -200 C.Polycarbonates such as available under the name Delrin can be used forliquified nitro gen, argon and carbon dioxide, water and petroleum at atemperature ranging from t-100 C. to -200 C. The last-mentioned fourresins are inexpensive but have somewhat small varieties ofapplications. Inexpensive polyvinyl chlorides can be used for variousoils and liquified carbon dioxide at a temperature ranging fro-m +50 to50 C. In addition to the above mentioned synthetic resins, glasses andceramics can be used for water, various oils and liquified hydrogen,nitrogen, oxygen, argon, chlorine, ammonia, butane and propane at atemperature of from +200 to 200 C. and of from +1000 to 270 C.respectively. Such inorganic spacers are expensive but most stable. Ingeneral, any material for the spacer has preferably its dielectricconstant low as compared with the associated liquid to be measured.

It will be seen that with the spacers 3a, 3b, 3c along a helical line atsubstantially equal intervals, there will be no more than any one spacerat any level along the space between the electrodes 1 and 4, andconsequently the cross-sectional area of the liquid in the space betweenthe electrodes will only be diminished by a single spacer at any pointalong the length of the electrodes. This arrangement accordinglyminimizes the efiect of the presence of the spacers on the measurementof the liquid level.

As an example, an aluminum circular tube having a length of 1280 mm. andan inside diameter of 7.8 mm. and an aluminum circular tube having anoutside diameter of 6 mm. were used as the external and internalelectrodes respectively to form an electrode assembly like that shown inFIG. 2 with a spacing therebetween of 0.9

as viewed from one end of the electrodes being positioned at angulardistances of 120 degrees on the profile thereof. The electrode assemblythus prepared was used to measare a level of liquified nitrogen whosedielectric constant is 1.43. The assembly exhibited an electrostaticcapacity of 270 pf. in the absence of liquified nitrogen and anincremental capacity of 9 pf. per 100 mm. of the liquid column. Theelectrode assembly disposed within a container of Dewar type having aninside diameter of 375 mm. and containing liquified nitrogen exhibitedan incremental capacity of approximately 8.75 pf. per 10 kg. of thecontent.

The invention has several advantages. For example, one of thecylindrical electrodes on which the mounting recesses are formed. isless decreased in mechanical strength because they are not locallyconcentrated on the particular portion of the one electrode. Further thespacers are immersed one after another into a liquid to be measuredresulting in small error in measurement. since the external electrode issupported by the internal electrode at a plurality of points, theelectrode assembly is prevented from resonating to any externalvibration and more particularly to any bending vibration, and further isetfectively damped. In addition, the spacing between both electrodes isstably maintained and more or less curved circular tubes may be used asthe electrodes without the necessity of straightening the tubes.

While the invention has been described in conjunction with a fewembodiments thereof it is to be noted that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. For example, instead of a rivet-shaped spacer, short rodshaving a predetermined common length may be inserted into a groove whosedepth is predetermined. Also the recesses or grooves may be formed onthe external electrode.

What we claim is:

An electrical liquid level gauge consisting essentially of a pair ofcoaxial cylindrical electrodes, and a plurality of spacers forseparating said pair of coaxial cylindrical electrodes from each other,each of said spacers being of an insulating material selected from thegroup consisting of tetrafluoroethylene, copolymers ofhexafluoropropylene, and tetraflnoroethylene, polyethylenes, polyamides,polyesters, polycarbonates, polyvinyl chlorides, glasses and ceramics,and being in the form of a rivet having a head portion of apredetermined fixed axial dimension corresponding to the radial distancebetween the internal cylindrical surface of the external electrode andthe external References Cited by the Examiner UNITED STATES PATENTS2,759,134 8/1956 Sullivan 3l7246 2,945,165 7/1960 Franzel 3172462,982,895 5/1961 Exon 3l7246 ROBERT K. SCHAEFER, Primary Examiner. JOHNF. BURNS, Examiner.

a E. GOLDBERG, Assistant Examiner.

